The invention relates to a arrangement for reading an information carrier, including a read head for scanning the information carrier along a scanning path and thus generating one or more electric signals in response to a physical pattern recorded along the scanning path, electrical conductors for conveying the one or more electric signals to a signal processing unit.
Such an arrangement is described in U.S. Pat. No. 5,103,439. The arrangement disclosed therein is suitable for reading an optical information carrier. The read head has a radiation source for the generation of radiation, means for imaging the radiation onto a scanning spot on the information carrier and for imaging radiation that leaves the scanning spot onto a detector. The detector has a first to fourth sub-detector and a current-voltage converter for the generation of voltage signals in response to current signals generated by the sub-detectors. In practical versions of the known arrangement the transimpedance amplifier is coupled to a signal processing unit via a flexible cable. From the signals supplied by the current-voltage converter this unit derives an information signal, as well as error signals for servo systems for the control of the position of the read head and of the focusing of the scanning spot.
At high read speeds, for example 10 to 20xc3x97DVD, disturbing reflections may appear at the input of the signal processing unit, as a result of which the signals conveyed via the flexible cable are unreliable. The reflections can be counteracted by terminating the respective conductors of the flexible cable with a resistive impedance near the input of the signal processing unit. However, this gives rise to a substantial increase of the load of the current-voltage converter.
It is an object of the invention to provide an arrangement of the type defined in the opening paragraph, which is suitable for comparatively high reading speeds and in which the load of the current-voltage converter is yet limited.
To this end, the arrangement in accordance with the invention is characterized in that the electrical conductors include one or more signal conductors and at least one reference conductor, the one or more signal conductors being coupled, near the signal processing unit, to the at least one reference conductor via a series connection of a resistive impedance and a capacitive impedance.
In an arrangement for reading an optical information carrier the signals generated by the current-voltage converter have both low-frequency components and high-frequency components. The first-mentioned components are of importance for the servo systems for radial and axial control. The last-mentioned components are indicative of the information read from the information carrier.
In the arrangement in accordance with the invention the impedance formed by the series connection of the resistive and the capacitive impedance is comparatively low at high frequencies. On the one hand, this results in a correct termination for the high-frequency components. On the other hand, the series connection has a comparatively high impedance for the low-frequency components, as a result of which the load of the current-voltage converter is limited.
An attractive embodiment of the arrangement in accordance with the invention has the characteristic feature defined in claim 2. In practice, the signals from the read head are transferred to the signal processing unit with the aid of various signal conductors. The inventor has found that it is possible to use a common capacitive impedance without any appreciable loss in signal quality. This enables the number of component parts to be reduced.
A practical embodiment of the arrangement in accordance with the invention is characterized as defined in claim 3. The read head generally includes a preamplifier, for example in the form of a transimpedance amplifier, also referred to herein as current-voltage converter. In the embodiment defined in claim 3, each signal conductor is disposed adjacent an auxiliary conductor, which itself does not conduct a signal. This limits electromagnetic interference.
This embodiment is particularly favorable in the variant defined in claim 4. In this variant of the detector a radial error signal can be derived by means of the DPD method. In said method the signals corresponding to the first and the third sub-detector are added to one another. Then, it is not a problem if there is crosstalk between these signals in the cable with the conductors. The same applies to the signals from the second and the fourth sub-detector.